why can't Forza sell like Gran Turismo

[Oyvoyvoyv]

One of the reasons, is that racing games appeal to pretty much everyone. Therefore, a bigger installbase does in this case mean a lot.

Another thing, is that when GT came out, it crushed everything quality wise. Even if the opponents have managed to catch up, people will still buy GT. This is the same case as GTA, or Final Fantasy.

So really, GT is a case of its own. For comparing racing games sales, it and Mario Kart are simply too good, so you should compare those two, and then the rest (ala, NFS, Forza and such).